Anode effect termination



N 1970 L, s. NEWMAN ETAL 3,539,461

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SEQUENCE or OPERATION (1.) R CLOSES-TiMER STARTS (2.) Tu CLOSE-Tlb OPENS-TIMER SEALS |N 'M/WWRS (3.) T6. CLOSE-TGb OPEN -WHIT LIGHT LEON NEWMAN (4.) Tu oPeN Tlb CLOSE -TmR STOPS BY JACK E. GRIFFIN (5.) a CLOSED-TIMER STARTS (6.) 10 OPEN -T6b CLOSE -TIMER STOPS .1. (7.) a OPENEN -SYSTEM IS RESET TATTORNEY United States Patent 01 fee ANODE EFFECT TERMINATION Leon S. Newman and Jack E. Griffin, Ravenswood, W. Va., assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed Oct. 19, 1967, Ser. No. 676,500

Int. Cl. C22d 3/12; B01k 3/00; C22d 3/02 US. Cl. 20467 10 Claims ABSTRACT OF THE DISCLOSURE The instant invention relates to a process for terminating an anode effect in an electrolytic cell for the production of aluminum and a system useable in carrying out the process. The process involves determining when the voltage drop across the cell exceeds about 150 percent of the normal operating value and lowering the cell anode so as to reduce the anode-cathode distance in the cell to from about 30 to about 60 percent of the normal operating distance. The available alumina concentration in the bath or electrolyte is adjusted to from about 2 percent to about 6 percent by weight. The anode is raised so as to restore the normal anode-cathode distance and the anode effect is terminated.

BACKGROUND OF THE INVENTION The metal aluminum is extracted from aluminum-bearing compounds such as alumina (A1 by electrolysis from a molten salt bath or electrolyte. In the production of aluminum by the conventional electrolytic process, commonly referred to as the Hall-Heroult process, the electrolytic cell comprises in general a steel shell having disposed therein a carbon lining. The bottom of the carbon lining, together with a layer of electrolyticallyproduced molten aluminum which collects thereon during operation, serves as the cathode. One or more consummable carbon electrodes is disposed from the top of the cell and is immersed at its lower extremity into a layer of molten electrolyte which is disposed in the cell. In operation, the electrolyte or bath which is a mixture of alumina and cryolite is charged to the cell and an electric current is passed through the cell from the anode to the cathode via the layer of molten electrolyte. The alumina is dissociated by the current so that aluminum is deposited on the liquid aluminum cathode and oxygen is liberated at the carbon anode, forming carbon monoxide and carbon dioxide gas. A crust of solidified electrolyte and alumina forms on the surface of the bath, and this is usually covered over with additional alumina.

In the conventional electrolytic process, use has been made of two types of electrolytic cells, namely that commonly referred to as a prebake cell and that commonly referred to as a Soderberg cell. With either cell, the reduction process involves precisely the same chemical reactions. The principal difference is one of structure. In the prebake cell, the carbon anodes are prebaked before being installed in the cell, while in the Soderberg, or self-baking anode cell, the anode is baked in situ, that is, it is baked during operation of the electrolytic cell, thereby utilizing part of the heat generated by the reduction process. The instant invention is applicable to either cell.

3,539,461 Patented Nov. 10, 1970 A typical aluminum electrolytic bath used in commercial installations might have the following composition: 1 to 10 percent alumina, 0 to 10 percent aluminum trifluoride, 5 to 12 percent calcium fluoride, and to percent cryolite. As the electrolysis continues, alumina is consumed in direct proportion to the metal production. As the alumina concentration in the electrolyte is reduced, a point is reached where a troublesome phenomenon known as an anode effect occurs. The voltage drop across the cell can increase, for example, from around 4 volts to as much as 40 volts and even higher. This effect is generally attributed to too low a concentration of alumina in the reduction cell bath or electrolyte. The actual concentration of alumina in the electrolyte at which this effect occurs seems to depend upon the temperature, the composition of the electrolyte and the anode current density. The occurrence of the anode effect is the signal for the addition of more alumina. The attendant does this by breaking the frozen crust on top of which he has previously distributed a layer of alumina. The addition of the alumina, as well as a vigorous stirring of the electrolyte, causes the anode effect to disappear, after which the electrolysis continues its normal course until the next anode effect occurs.

There are several disadvantageous results of an anode effect such that terminating an effect as quickly as possible and restoring normal operating conditions is highly desirable. During the normal course of electrolysis, the bottom operating surface of the anode is surrounded by gas bubbles which are constantly escaping from it. They appear to form on the anode, break away easily, and escape from the electrolyte. Smooth evolution of gas surrounding the anode is a sign of normal operation. The moment the anode effect occurs, the bottom operating surface of the electrode seems to be entirely surrounded by a film of gas. This covers the surface of the anode and pushes the fused electrolyte or bath away, producing the so-called non-wetting of the anode. Small arcs form between the electrolyte and the anode. Complete interruption of the current does not occur, as some current is being carried by the continually shifting arcs. The arcs cause local heating, volatilizing some bath material but producing sufficient gas so that individual arcs are almost immediately broken. New arcs form, as the bath film near the anode must necessarily be uneven in character, and momentary contacts take place between the anode and bath. This overheating causes very rapid consumption of the anode. A very important undesirable result of the anode effect is a large unproductive power consumption.

The prior art describes, in addition to the manual procedure mentioned above, many attempts to rapidly control or terminate anode effects. Some of these prior art proposals have been quite exotic which is indicative of the magnitude of the problem and the extreme methods to which those skilled in the art have been willing to resort in order to control this situation.

British Pat. 853,056 describes a procedure utilizing a sonic vibrator to terminate anode effects. How this procedure terminates the anode effect is not stated. Louis Ferrand, in US Pat. 2,560,854, describes a procedure for terminating an anode effect which involves slowly oscillating the anodes from a lean region in the electrolyte to a richer region (richer in alumina). Allegedly, this mixes the alumina in the bath more thoroughly and assists 3 in terminating an anode effect. The cumbersome equipment involved to carry out this procedure is easily imagined. Presumably the oscillatory action of the anode facilitates the escape of gaseous bubbles from beneath them. More even current density for the cell is also alleged to be a result of this interesting operation.

Robert J. Cooper, in U.S. Pat. 2,930,746, discusses the manual procedure mentioned above and notes other practices which can be used, include raking the metal in the aluminum pot below the anodes vigorously to cause temporarary short-circuits from the anode to the aluminum. This cools the gases and disturbs them so they escape more readily. Lowering of the anodes closer to the molten aluminum pad is also used to stop the anode effect. He then proposes a voltage, current and temperature responsive control system which will raise and lower the anode as needed to maintain these variables at predetermined operating levels. According to the patent, this procedure will also terminate anode effects.

SUMMARY OF THE INVENTION In comparison with these prior art procedures, the instant invention provides a simple yet effective procedure for rapidly terminating anode effects. If the practices of this invention are adhered to, better than 90 percent of all anode effects can be rapidly terminated. The process involves determining when the voltage drop across the cell exceeds about 150 percent of the normal operating value and when that level has been reached, lowering the cell anode so as to reduce the anode-cathode distance in the cell to from about to about 60 percent of the normal operating distance and desirably to not more than about 51 percent. The available alumina concentration in the bath is adjusted to from about 2 percent to about 6 percent by weight of the bath. The anode is raised so as to restore the normal anode-cathode distance and when raised the anode effect is terminated and normal operating conditions are restored.

The available alumina concentration in the bath or electrolyte may be adjusted by placing a quantity of alumina on the crust over the cell electrolyte and breaking the crust on the cell electrolyte so as to cause the alumina placed thereon to feed into the electrolyte. Various procedures may be used for the alumina addition. The alumina may be fed into the electrolyte prior to the lowering of the cell anode, during the lowering of the cell anode, after the lowering of the cell anode, or even after the raising of the cell anode. A desirable procedure is to feed the alumina into the electrolyte in increments so as to gradually introduce the alumina into the bath and thereby facilitate dissolution thereof and prevent formation of muck on the cathode, in which case at least one of the increments of alumina is fed into the electrolyte after the lowering of the cell anode. By muck is meant an accumulation of alumina and other bath constituents which can collect between the carbon lining and molten aluminum pad and interfere with the operation of the cell and the purity of the metal produced.

An appropriate system for terminating an anode effect in accordance with the instant invention involves first of all, suitable means such as a voltmeter for measuring the voltage drop across the cell and signalling when the voltage drop across the cell exceeds about 150 percent of the normal operating value. This signal then triggers the system into action.

Means, such as an anode jacking motor, in response to the voltage signal lower the cell anode or anodes so as to reduce the anode-cathode distance in the cell to from about 30 to about 60 percent of the normal operating distance. The available alumina concentration in the cell is adjusted by other means responsive to the voltage signal to from about 2 percent to about 6 percent b weight. Finally, means are provided for raising the anode so as to restore the normal anode-cathode distance.

4 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of the cumulative percent of anode effects terminated versus the remaining percent of normal anode-cathode distance after anode lowerin FIG. 2 is a schematic circuit diagram of a possible anode effect terminator system.

FIG. 3 shows the cam timer sequence for the anode effect terminator system shown in FIG. 2.

DETAILED DESCRIPTION A series of tests were run in developing the instant invention. The cells used in the tests were commercial 72,- 000 ampere cells operating at about 5 volts. In all of the tests, a voltmeter was used to measure the voltage drop across the cell. In the initial tests, when the voltage drop across the cell exceeded about 130 percent of the normal operating value, a motor was actuated to lower the anode so as to reduce the anode-cathode distance to about percent of the normal value. Feeders were operated six times at IO-second intervals for the onset of the anode effect to feed approximately 30 pounds of alumina to the cell, increasing the concentration to from about 2% to about 6% by weight, with the anodes having been lowered within 19 seconds after the anode effect occurred. A puncher bar was operated after the alumina was added to feed the alumina, which had been placed on the crust over the cell electrolyte by the feeders, into the electrolyte or bath. The anode jack motor was then operated to raise the anode and bring the anode-cathode distance to near normal adjustment. This result is shown at point A in FIG. 1.

In the next tests, the anode jack motor was operated so as to lower the cell anode to reduce the anode-cathode distance in the cell to not more than about percent of the normal operating distance. This was done within 15 seconds after the signal that the voltage drop across the cell had exceeded about percent of the normal operating value. The feeders were operated 12 times after the start of the anode effect in 10-second intervals so as to place a measured quantity of alumina (approximately 60 pounds) on the crust over the cell electrolyte. The crust breaker puncher 'bars were operated after 6 feeder operations and again after 12 feeder operations, breaking the crust on the cell electrolyte so as to cause the alumina placed thereon to feed into the electrolyte. The anode jack shaft motor was operated 100 seconds after the start of the anode effect to raise the anodes so as to substantially restore the normal anode-cathode distance. Of 333 recorded anode effects, 74 percent were successfully terminated, as shown at point B in FIG. 1.

In the next series of tests, the anode-cathode distance reduction was increased so that the anode-cathode distance was reduced to not more than about 41 percent of the normal operating distance. The crust breaker puncher bar was operated once after each three feeder operations. The anode-cathode distance was left at the reduced value for about 155 seconds before the anode was raised so as to substantially restore the normal anode-cathode distance. In these tests and in the following tests, the system was actuated when the voltage drop across the cell exceeded about percent of the normal operating value. Of 81 recorded anode effects, 77 or about 95 percent were successfully terminated. This is recorded at point C in FIG. 1.

In the next tests, the anode-cathode distance adjustment was the same as for those tests indicated at point B on FIG. 1. Differences between Tests B and these tests, Tests D, are as follows: the feeders were operated 17 times to feed 85 pounds of alumina. The crust breaker puncher bar was operated 5 times and the anode-cathode distance was left at the reduced value for seconds. Of 90 recorded anode effects, 84 or about 93 percent were successfully terminated as recorded at Point B in FIG. 1.

Tests E duplicated the features of Tests C, except that the anode jack shaft motor was operated so as to reduce the anode-cathode distance to about 51 percent of the normal anode-cathode distance and the crust breaker puncher bar was operated 5 times. This procedure successfully terminated over 95 percent of all anode effects experienced as recorded at point B in FIG. 1.

During a further exploratory test period to determine the desired reduction in anode-cathode distance for consistent termination of anode effects, 97 percent of the anode effects were terminated. The minimum reduction in anode-cathode distance necessary to successfully terminate an anode effect was to not more than 81 percent of the normal operating distance. Most of the anode efiects were successfully terminated when the anodecathode distance was reduced to not more than about 40 percent of the normal operating distance and 99% were successfully terminated when the distance was reduced to not more than about 30% of the normal operating distance. The data from these tests were used to plot the curve shown in FIG. 1.

The schematic diagram for an anode effect terminator system for carrying out the process of this invention is shown in FIG. 2 and FIG. 3 shows the cam timer sequence for the system. This system will lower the cell anode so as to reduce the anode-cathode distance in the cell to from about 30 to about 60 percent of the normal operating distance and desirably to not more than about 51% of the normal operating distance. The available alumina concentration in the bath is adjusted to the desired value in the range of from about 2 percent to about 6 percent by weight by feeding the alumina in increments, shown in the example as 17 five-pound increments in which the first increment of alumina is placed on the crust almost simultaneously with the lowering of the anode and the last increment being added as the anode is raised. The crust on the cell electrolyte is broken by the crust breaker puncher bar 5 times during this sequence of incremental feeding of alumina. The system is designed to be actuated when the voltage drop across the cell exceeds about 150 percent of the normal operating value. At this point, the timer motor is actuated and the sequence of operations described and shown in the schematic and cam timer sequence takes place. This procedure at the S 1% of normal anode-cathode distance setting will extinguish at least 95 percent of all anode effects incurred in normal reduction cell or pot operation.

From these series of tests and the data presented, it can be seen that the signficant factor in terminating anode effects by reducing anode-cathode distance is that the final anode-cathode distance is reduced enough to cause the effect to cease. Reducing the anode-cathode distance to not more than about 51 percent of the normal operating distance has been found to do this in 95 percent of the cases. The minimum anode-cathode distance reduction that provides acceptable reliability, i.e., 95 percent, is preferred to minimize the disturbance of the bath level. The addition of alumina to the bath as part of the terminating procedure aids in anode effect terminations. The added alumina, if it is dissolved in the bath, favors return of the cell to normal electrolysis. The amount of alumina added to the bath should be no more than the bath can dissolve to prevent formation of muck on the cathode. The anode effect terminator system and the process de scribed herein charged only enough alumina during terminating procedures to prevent recurring anode effects. The improved alumina regulation provides the means to keep the cathode free of muck. The elimination of muck on the cathode results in improved operating efficiency.

The crust cover over the bath of a cell or pot having an anode effect will remain almost completely intact if the anode effect is quickly controlled and if an excessive anode adjustment is not made. With prompt anode effect termination, the heat balance throughout the line of reduction cells remains stable and less attention must be paid to the cells to attain good cell performance. The value TABLE I.INCREASE I EAERAGE LINE LOAD WITH Line operation without anode effect terminators AE/Pot. Amperes KwJPot. day

Line operation with anode effect terminators Average Amperes Kw./Pot.

The increase in the average amperage of 474 amperes shown in this table is made without an increase in power cost, and with a reduction in potroom labor requiremerits. This improvement in average potline load is possible because anode effects are promptly and successfully terminated.

Prompt successful anode effect termination in accordance with this invention saves hard physical effort normally required from pot men. This reduction in the need for hard work reduces the amount of manpower required to operate a potline. The work assignment of the pot men can be carried out without unscheduled interruptions to terminate anode effects. Less than 1 anode effect out of 20 will require manual termination. Only a few of the manually terminated anode effects require more than a small amount of the raking procedure mentioned above in the discussion of the Cooper patent. Pot men will be able to work at other duties without stopping to terminate an anode effect.

While there have been shown and described hereinabove possible embodiments of this invention, it is to be understood that the invention is not limited thereto and that various changes, alterations, and modifications can be made thereto without departing from the spirit and scope thereof as defined in the appended claims wherein:

What is claimed is:

1. The process of terminating an anode effect in an electrolytic cell for the production of aluminum which comprises:

(a) when the voltage drop across the cell exceeds about percent of the normal operating value, lowering the cell anode so as to reduce the anodecathode distance in the cell from normal operating distance to from about 30 to about 60 percent of the normal operating distance;

(b) adjusting the available alumina concentration in the bath to from about 2 percent to about 6 percent by Weight;

(c) raising the anode so as to restore the normal anode-cathode distance, whereby the anode effect is terminated.

2. The process of claim 1 wherein the available alumina concentration in the bath is adjusted accordingto the following procedure:

(a) placing a quantity of alumina on the crust over the cell electrolyte; (b) breaking the crust on the cell electrolyte so as to cause the alumina placed thereon to feed into the electrolyte.

3. A system for terminating an anode efiect in an electrolytic cell for the production of alumina which comprises:

(a) means for measuring the voltage drop across the cell and signalling when the voltage drop across the cell exceeds about 150 percent of the normal operating value;

,(b) means responsive to the voltage signal for lowering the cell anode so as to reduce the anode-cathode distance in the cell from normal operating distance to from about 30 to about 60 percent of the normal operating distance;

() means responsive to the voltage signal for adjusting the available alumina concentration in the cell to from about 2 percent to about 6 percent by weight;

(d) means for raising the anode so as to restore the normal anode-cathode distance, whereby the anode effect is terminated.

4. The process of claim 2 wherein the alumina is fed into the electrolyte prior to the lowering of the cell anode.

5. The process of claim 2 wherein the alumina is fed into the electrolyte after the lowering of the cell anode.

6. The process of claim 2 wherein the alumina is fed into the electrolyte after the raising of the cell anode.

7. The process of claim 2 wherein the alumina is fed into the electrolyte in increments so as to facilitate dissolution thereof and to prevent formation of muck on the cathode.

8. The process of claim 7 wherein at least one of the increments of alumina is fed into the electrolyte before the lowering of the cell anode and at least one of the increments of alumina is fed into the electrolyte after the lowering of the cell anode.

9. The process of claim 2 wherein the alumina is fed into the electrolyte during the lowering of the cell anode.

10. The process of claim 1 wherein the cell anode is lowered so as to reduce the anode-cathode distance in the cell to not more than about 51 percent of the normal operating distance.

References Cited UNITED STATES PATENTS 3,455,795 7/1969 Boulanger et al. 204-67 3,317,413 5/1967 Chambran 204-67 3,329,592 7/1967 Uhrenholdt 20467 3,400,062 9/1968 Bruno et a1 204-243 XR 3,434,945 3/ 1969 Schmitt et al. 20467 FOREIGN PATENTS 1,397,946 3/ 1965 France.

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 2'04--225, 245

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,539,461 Dated November 10, 1970 Inventor(s) Leon S. Newman and Jack E. Griffin It is certified that: error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 21, "for" should be from Column 6, line 15, ".57" should be 51 16, ".49" should be .47 17, ".47" should be .49 18, ".45" should be 47 and 20, ".481" should be -:485--.

Signed and sealed this 18th day of May 1 971 (SEAL) Attest:

EDWARD M.FLEICI-ER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

